The invention relates to techniques for creating digital images by compositing graphics elements.
A digital image is generally defined by color and alpha information. The alpha is optional. The color may be defined in any of a variety of color spaces, including a monochrome grayscale space. The image definition may be in a raster format, a vector format, or a combination of them.
In a digital graphics processing computer program, the traditional compositing loop has the following conceptual structure:
1. Initialize an accumulation buffer.
2. For each element to be composited in order,                use the element to modify the accumulation buffer.        
This loop-based model corresponds closely to a layered compositing model. Other compositing models are based on trees or directed-acyclic graphs of compositing operators that combine a collection of leaf images to produce a final image. Such non-linear models are very different in their presentation to the user and the present invention is not relevant to them. In layered compositing, an element can be an object, a layer, or a group of layers or objects or both. A description of layered compositing can be found, for example, in commonly-owned U.S. Pat. No. 6,028,583.
The invention involves a reformulation of what happens when a compositing loop uses an element to modify the accumulation buffer. Every graphics program based on a layered compositing model must define how elements modify the accumulation buffer, but such definitions have been somewhat ad hoc. Examples include blending individual layers; blending a group by making a copy of the accumulation buffer, blending the grouped layers into the copy, and then using the masks for the group to crossfade the result into the accumulation buffer; blending clipping groups; and applying color adjustments by using adjustment layers.